American scientists found that mice regularly exposed to light at night became ‘depressed’ - showing less interest in doing ‘fun’ things, being less likely to explore new objects in their cages and not moving around as much. They also had higher levels of cortisol, the stress hormone.

Samer Hattar, professor of biology at Johns Hopkins University in the US, said: “Basically, what we found is that chronic exposure to bright light - even the kind of light you experience in your own living room at home or in the workplace at night if you are a shift worker - elevates levels of a certain stress hormone in the body, which results in depression and lowers cognitive function.”